Several techniques have evolved for dealing with the problems associated with detecting objects in a light-scattering medium. Many of these techniques and their drawbacks are discussed in the above-referenced U.S. Pat. No. 5,467,122.
Other techniques include using an imaging system based on one or more analog-to-digital converters (ADCs). In such a system, light is first directed toward an object to be imaged.
Light that is reflected back from the object is then directed to a comparator. If the comparator detects a requisitely large signal, a state change is produced which in turn triggers an ADC to direct a signal to a streak or charge-coupled device (CCD) camera for imaging.
The drawbacks of this technique, however, are multifold. If a single ADC is used in the system, it may entirely miss a multiplicity of signals reflected back from juxtaposed objects. It may, instead, form only a partial image of such objects based on just the leading edge of a signal waveform.
Additionally, use of a single ADC restricts the imaging to very small area coverage. Furthermore, even a single ADC is typically a large bulky system and generates a great amount of heat. Thus, even if several ADCs are used collectively to effectively increase the range, both the size and heat accumulation issues remain to be overcome. Dealing with these issues adds to the cost of producing such systems, as does the fact that the applicable ADC systems are not typically prefabricated. Most of the commercially available ones are for physically small arrays or for very-large-scale integration (VLSI).
Another problematic technique is commonly referred to as the “Magic Lantern”. For this technique, several intensified CCD cameras are used to define range bins. Each camera is dedicated to a separate piece of time referred to as a trigger image point.
This approach, however, leads to poor range resolution and area coverage—especially for objects that are spaced far apart along the range direction. As discussed at length in the previously mentioned '122 patent, this is basically another range-gating technique. Like the ADC system, it has the drawbacks of poor ability to see clustered detail or to acquire multiple events over a single lidar pulse.
Streak-tube lidar systems are generally based on the generation of a periodic series of discrete pushbroom-shaped pulse beams to illuminate an object in semiturbid medium. When reflected back, the pulses are collected through a slit and onto a streak tube—which is in turn coupled to an imaging detector such as a CCD for imaging.
Such streak-tube lidar systems overcome many of the aforementioned problems, but are subject to certain drawbacks. The streak tube itself is a complex, bulky, expensive and relatively fragile vacuum-tube device, requiring high voltages for both basic operation and control.
As will be seen, these characteristics impose severe limitations upon any effort to generate or use images in other formats or for different purposes. The possible existence of such variations in format and purpose are themselves considered part of the present invention; hence these configurations will be introduced in a later section of the present document.
What is needed, to realize such novel configurations and purposes, is an imaging system that provides an accurate and reliable image of an object in a light-scattering medium—and that not only eliminates the problems associated with range-gating techniques and bulky, heat-generating ADCs but is also relatively inexpensive to produce as well as compact and more-easily transportable. Important aspects of lidar imaging thus remain amenable to useful refinement.